| Abstract: | Summary The interplay between support systems and the rock when tunneling under squeezing conditions is normally studied by means
of two-dimensional analyses. The present paper shows that the underlying plane strain assumption involved in a two-dimensional
analysis may lead, under certain conditions, to ground pressure and deformation values that are considerably lower than those
produced by stress analyses that take into account spatial effects in the vicinity of the tunnel face. The differences are
due to the stress path dependency in the elasto-plastic behavior of the ground and, more specifically, to the inability of
the plane strain model to map the actual radial stress history, which involves a complete radial unloading (and, later, a
re-loading) of the tunnel boundary over the unsupported span. This inherent weakness of any plane strain analysis is relevant
from the design standpoint, particularly for heavily squeezing conditions that require a yielding support. For the majority
of tunneling conditions and methods, however, involving as they do the completion of a stiff support within a few meters of
the face, the errors introduced by the plane strain assumption are not important from a practical point of view. |
